1. Field of the Invention
The present invention relates generally to a stacked-layer structure polysilicon emitter contacted diode fabrication processes, and more specifically, to a polysilicon emitter diode formed by a stacked-amorphous-silicon film (SAS) or a stacked-polycrystalline-silicon film (SPS).
2. Background of the Invention
A conventional diode having a shallow polysilicon emitter contacted p-n junction is shown in FIG. 1. The conventional diode will be hereinafter referred to as an as-deposited poly-Si (ADP) diode, which includes a p-type substrate of silicon 10, a single layer of heavily doped n.sup.+ -type polysilicon 30, and an aluminum layer 40 used as the electrode. Between the n.sup.+ -type polysilicon layer 30 and the p-type silicon substrate 10 is there formed a n.sup.+ -p junction 11, which has a shallow depth of about 0.2 .mu.m. Since the n.sup.+ -p junction 11 is buried into the p-type substrate 10, it is customarily termed as "an polysilicon emitter contacted p-n junction".
The process for fabricating the conventional ADP diode of FIG. 1 includes in general a first step of depositing a layer of pure polysilicon upon the p-type substrate 10 and a second step of implanting donor impurities such as arsenic ions or phosphorus ions onto the pure polysilicon layer to form the n.sup.+ -type polysilicon layer 30 and a final step of forming the aluminum layer 40. At the second step, the ion implantation process is controlled in such a manner that donor ions penetrate the p-type substrate 10 and whereby a shallow n.sup.+ -p junction is formed. The implantation is controlled in such a manner that the donor ions diffuse to a predetermined extent, i.e. the depth of the n.sup.+ -p junction, into the p-type substrate.
The drawback of the conventional ADP diode of FIG. 1 is that undesired thin layer of native oxide formed between the n.sup.+ -type polysilicon layer 30 and the p-type substrate 10 is difficult to break up completely. As a consequence, there is caused a nonuniform junction and an unstable electrical characteristics of the polysilicon emitter contacted device.